Spatial, Temporal, and Depth Distributions of Mercury Contamination in the Penobscot River, Maine
Sloane Hauck, Julia Kirkland, Meg O’Brien, and Cira Ortiz
Background
The Penobscot River in Maine has been the object of environmental concern and focus for years. It is the largest watershed in New England and hosts the largest run of Atlantic salmon on the East Coast (Natural Resources Council of Maine, 2013; The Nature Conservancy, n.d.). Despite its ecological importance, the watershed has experienced significant degradation due to high levels of mercury contamination by a chemical plant. In 1967, a pharmaceutical company, Mallinckrodt began discharging mercury into the river as part of its production process, leaving 13 tons of Mercury in the river by 1970 (Island Institute, n.d.).
Mercury is a particularly worrisome and challenging contaminant because it does not easily break down. The contamination has had a significant and lasting impact on the surrounding ecosystem and nearby communities. The state of Maine issued an advisory warning pregnant women against eating certain species harvested from the river, and certain lobster and crab fisheries have been shut down (Maine Department of Environmental Protection, n.d.).
However, due to the steadfast and tenacious efforts of environmental advocates, this story has a silver lining. Senior attorney Nancy Marks from the NRDC (the Natural Resources Defense Council) filed a lawsuit in 1998, and after two decades of litigation, a major legal breakthrough was achieved. In 2022, the district attorney of Maine settled the long-running case, requiring Mallinckrodt to pay $197 million in remediation for the mercury damage (A 22-Year Court Battle Ends with Justice for the Penobscot River, 2022; Penobscot River Remediation, n.d.).
Because this watershed has been the subject of extensive study to document the need for remediation, there is considerable data about its sediment history. NOAA has compiled datasets on sediment, soil, and tissue chemistry from various studies of the Penobscot watershed (Island Institute, n.d.). Using this data, we will examine mercury levels in the sediment.
Research Questions and Hypotheses
Question 1: How do mercury contamination levels change over time in the sediments in the Penobscot River?
Question 2: Does mercury contamination increase with depth?
Hypothesis 1: Post closure of the site, we would expect that mercury levels in surface sediment will decrease over time and as they get further away from the site due to natural attenuation.
Hypothesis 2: Over time, mercury concentrations have decreased in the Penobscot River due to sedimentation in the river.
Dataset Information
The dataset was generated from NOAA’s DIVER (Data Integration, Visualization, Exploration and Reporting) system, which is part of the NOAA Damage Assessment, Remediation, and Restoration Program (DARRP). The data contains Mercury results from sediment samples collected in the Penobscot River region in Maine from June 1st, 1970 to May 5th, 2021. Data is not available for every month of the years provided, likely due to shifting priorities with monitoring or project specific challenges.
| Item | Value |
|---|---|
| Data Source | NOAA DIVER |
| Date Range | June 1st, 1970 to May 5th, 2021 |
| Number of Records | 28,058 |
| Records with Coordinates | 24,501 |
Data Exploration
We began by importing the raw NOAA Diver dataset and subsetting it to include only the columns relevant to our analysis, including site location, depth, depth type, chemical type (mercury), and concentration. Next, we flagged all records containing coordinate information and created a new subset consisting only of samples with valid coordinates. This subset was then converted into a spatial dataset for future mapping and spatial analyses.
After cleaning the dataset, we plotted sample depths against mercury concentrations, mapped the sampling locations onto the Penobscot River, and examined concentration trends over time. We plotted a reference line representing the mercury probable effect concentration (PEC) (MacDonald et al., 2000), above which harmful effects to sediment-dwelling biota are likely to occur.
Concentrations of mercury in the Penobscot River by depth.
Concentrations of mercury in the Penobscot River over time. The red line represents the mercury PEC, which is 1.06 PPM (MacDonald et al. 2000).
A preliminary review revealed a major data gap between 1970 and 2000, leaving insufficient information to conduct a long-term time-series analysis or evaluate the direct effects of the 1967 contamination event. This gap reflects broader historical issues in environmental monitoring and regulatory oversight during that period. To ensure analytical consistency, we restricted our final dataset to samples collected from 2000–2021.
Data Wrangling
Our initial exploration showed that some concentration results were coded as –9, NOAA’s indicator for missing data, so these records were excluded. We also identified two measurement units—PPM and mg/L—but because mg/L is not appropriate for solids and appeared in only one record, that entry was removed. Additionally, some samples had missing or inconsistent depth information, so an average depth value will need to be generated for further analysis.
Spatial Analysis
The analysis begins with orienting the viewers to the Penobscot River in Maine, in relation to mercury concentrations. The map was created using the shapefiles that were read in, and the subsequent spatial dataset subset during the original data wrangling. In the figures you can see that the highest concentrations are centered around the historical site itself.
This first map shows mercury concentration, in part per million (PPM) as a color gradient in the spatially situated river. The highest measured concentrations reach 108.63 PPM, about 100 times the PEC of mercury in sediment, 1.06 PPM. Because the PEC represents the threshold at which benthic macroinvertebrates are likely to experience toxic effects from mercury, these results emphasize how severely toxic the Penobscot River sediments are.